Colistin represents one of the very few available drugs for treating infections caused by carbapenem resistant Enterobacteriaceae (CRE). As such, the recent plasmid-mediated spread of the mobilized colistin resistance gene mcr-1 poses a significant public health threat requiring global monitoring and surveillance. In this work, we characterize the global distribution of mcr-1 using a dataset of 457 mcr-1 positive sequenced isolates consisting of currently publicly available mcr-1 carrying sequences combined with an additional 110 newly sequenced mcr-1 positive isolates from China. We find mcr-1 in a diversity of plasmid backgrounds but identify an immediate background common to all mcr-1 sequences. Our analyses establish that all mcr-1 elements in circulation descend from the same initial mobilization of mcr-1 by an ISApl1 transposon in the mid 2000s (2002-2008; 95% higher posterior density), followed by a dramatic demographic expansion, which led to its current global distribution. Our results provide the first systematic phylogenetic analysis of the origin and spread of mcr-1, and emphasize the importance of understanding the movement of mobile elements carrying antibiotic resistance genes across multiple levels of genomic organization.